13

What Youll Learn

How thunderstorms,tornadoes, and hurricanes form. What the effects ofsevere weather are. How repetitive weatherpatterns can causedroughts, floods, andother hazards.Why Its ImportantSevere weather can resultin extensive propertydamage and loss of life.To implement safety measures and make otherpreparations for severeweather, its necessary tounderstand when andwhere severe weather islikely to occur.

To find out more about

severe weather, visit theEarth Science Web siteat earthgeu.com

328

The Natureof Storms

Discovery LabDid you know that lightningcauses thunder? During a thunderstorm, lightning can reach temperatures of 30 000C. This extreme heatcauses the air around the lightning toexpand rapidly, then quickly cool andcontract. The rapid expansion of airgenerates sound waves heard as thunder. You can model thunder using apaper bag.1. Blow into a brown paper lunch baguntil it is full of air.2. Hold the top of the bag firmly inone hand and twist it so that theair inside is trapped. Take care notto tear the bag.

13.1OBJECTIVES

Identify the processes

that form thunderstorms. Compare and contrastdifferent types of thunderstorms. Describe the life cycle ofa thunderstorm.VOCABULARY

air-mass thunderstormsea-breeze thunderstormfrontal thunderstorm

Model Thunder3. Strike the bag sharply with yourother hand so that the bag breaks.CAUTION: Always wearsafety goggles in the lab.Observe What did you hearwhen the bag broke? How isthis similar to the thunder produced by a lightning bolt?Light moves much faster thansound. Knowing this, what canyou infer about the movementof a thunderstorm if theamount of time between whenyou see the lightning and hearthe thunder increases betweeneach lightning flash?

ThunderstormsAt any given moment, nearly 2000 thunderstorms are occurringaround the world. Most do little more than provide welcome reliefon a muggy summer afternoon. Some, however, grow into atmospheric monsters capable of producing hail the size of baseballs,swirling tornadoes, and surface winds of more than 160 km/h. Thesesevere thunderstorms can also provide the energy for natures mostdestructive storms: hurricanes. All thunderstorms, regardless ofintensity, have certain characteristics in common.

HOW THUNDERSTORMS FORM

In Chapter 11, you learned that under the right conditions, convection can cause a cumulus cloud to grow into a cumulonimbus cloud.You also learned that cumulonimbus clouds produce thunderstorms.What conditions are necessary for this to happen? For a thunderstorm to form, three conditions must exist. First, there must be anabundant source of moisture in the lower levels of the atmosphere.13.1 Thunderstorms 329

As this moisture condenses, it releases latent heat. The release of

latent heat keeps the cloud warmer than the air around it, which iscrucial in maintaining the upward motion of the cloud. Second,some mechanism must lift the air so that the moisture can condenseand release latent heat. Youll read about these mechanisms on thenext page. Last, the portion of the atmosphere through which thecloud grows must be unstable. In other words, the air must continueto cool with increasing altitude for the growing cloud to stay warmerthan the surrounding air. Recall that air can rise only if its warmerthan the air around it. If an air mass is stable, even the release oflatent heat will not keep that air warmer than the air around it. Theupward motion and growth of the cloud will stop.Limits to Growth If the three conditions just described are met,the air will keep rising, causing more moisture to condense and creating more latent heat. This process will continue until the rising airmeets a layer of stable air that it cannot overcome, or until the rate ofcondensation, which diminishes with height, is insufficient to generate enough latent heat to keep the cloud warmer than the surrounding air. This second factor limits most cumulonimbus clouds to aheight of around 18 000 m. Because of factors which youll learnabout later in this section, typical thunderstorms last only about 30minutes, and individual storms are only about 24 km in diameter.Figure 13-1 shows which areas of the United States experience themost thunderstorms annually.

Average Number of Thunderstorm Days Annually

Figure 13-1 Geography

and the movement of airmasses both play roles inmaking thunderstormsmost common in the southeastern United States.

5 10

101015

20

25

25

25

30

202530

30

25

40

2535

2520

15 15

25

30

30

303035

35

20

3525

35 35

352015

45

40

50

60606065

25

30

3540

354050

50

10

50

4550

6555

4550

60

510

Alaska

1520253040555060 50 45 40 35

60

407035

50653545403530

30

25

Hawaii

Source: National Climatic Data Center, NOAA

330 CHAPTER 13 The Nature of Storms

25

5550

7565 70

60 65

8080

Puerto Rico

60657075757070757580859050 707085807570

More than 7050 to 7030 to 5010 to 30Under 10

AIR-MASS THUNDERSTORMSEarlier you learned that some mechanism mustlift air through a growing cloud so that its moisWarm airture can condense and release latent heat.Thunderstorms are often classified according tothe mechanism that caused the air to rise. If theair rose because of unequal heating of Earthssurface within one air mass, the thunderstormis called an air-mass thunderstorm. Theunequal heating of Earths surface reaches itsmaximum during mid-afternoon. Thus, airmass thunderstorms are most common then.There are two common types of air-massthunderstorms. Mountain thunderstorms occurwhen an air mass rises as a result of orographiclifting, which, as you learned in Chapter 11,involves air moving up the side of a mountain.Sea-breeze thunderstorms are common along coastal areas duringthe summer, especially in the tropics and subtropics. Sea-breezethunderstorms are local air-mass thunderstorms caused in part byextreme temperature differences between the air over land and theair over water, as shown in Figure 13-2.

FRONTAL THUNDERSTORMSThe second main classification of thunderstorms is frontalthunderstorms, which are produced by advancing cold fronts and,more rarely, warm fronts. In a cold front, cold air pushes warm airrapidly up the steep cold-front boundary. This rapid upward motioncan produce a line of thunderstorms, sometimes hundreds of kilometers long, along the leading edge of the cold front. Cold-frontthunderstorms get their initial lift from the push of the cold air.Because they are not dependent on daytime heating for their initiallift, cold-front thunderstorms can persist long into the night.Less frequently, thunderstorms can develop along the advancingedge of a warm front. In a warm front, a warm air mass slides up andover a cold air mass. The boundary between the two air masses is notsteep; thus, the air rises gradually. However, if the warm air behindthe warm front is unstable and moisture levels are sufficiently high,a relatively mild thunderstorm can develop.

STAGES

OF

Cool air

Sea breeze

Figure 13-2 During the

day, cool air over the oceanmoves inland and creates asea breeze. The cool airforces warm air over theland to rise. The rising aircools and sinks, creating aconvection cell. These conditions can produce strongupdrafts that result inthunderstorms.

DEVELOPMENT

A thunderstorm usually has three stages: the cumulus stage, the

mature stage, and the dissipation stage. The stages are classifiedaccording to the direction in which the air is moving.13.1 Thunderstorms 331

12

11

11

10

10

Height (km)

Height (km)

12

7654

32 F

0 C

7654

58 km

AFigure 13-3 The cumulusstage of a thunderstorm ischaracterized mainly byupdrafts (A). The maturestage is characterized bystrong updrafts and downdrafts (B). The storm losesenergy in the dissipationstage (C).

32 F

0 C

815 km

Cumulus Stage In the cumulus stage, air starts to rise nearly vertically upward, as shown in Figure 13-3A. This creates updrafts,which transport moisture to the upper reaches of the cloud. Themoisture condenses into visible cloud droplets and releases latentheat. As the cloud droplets coalesce, they form larger and largerdroplets, which eventually fall to Earth as precipitation. This beginsthe mature stage of a thunderstorm.Mature Stage Precipitation in a thunderstorm is composed ofwater droplets that formed at high, cool levels of the atmosphere. Asthe precipitation falls, it cools the air around it. The newly cooled airis more dense than the surrounding air, so it sinks rapidly to theground along with the precipitation. This creates downdrafts. AsFigure 13-3B shows, the updrafts and downdrafts form a convectioncell that produces the gusty surface winds associated with thunderstorms. In the mature stage, nearly equal amounts of updrafts anddowndrafts exist side by side in the cumulonimbus cloud.Dissipation Stage The production of downdrafts is ultimately thethunderstorms undoing. The convection cell can exist only if there isa steady supply of warm, moist air at Earths surface. Once that supplyruns out, the updrafts slow and eventually stop. In a thunderstorm,shown in the photo on the next page, the supply of warm, moist airruns out because the cool downdrafts spread in all directions whenthey reach Earths surface. This cools the areas from which the storm

332 CHAPTER 13 The Nature of Storms

1211109

Height (km)

87654

0 C

32 F

321

811 km

draws its energy. Without the warm air, the updrafts cease and precipitation can no longer form. The storm is then in the dissipation stage,as shown in Figure 13-3C. This stage, which is characterized primarilyby lingering downdrafts, will last until the cloud runs out of previouslyformed raindrops. Next, youll explore the destructive forces that canbe unleashed when a severe thunderstorm strikes.

1. Its 2:00 A.M. in the northeastern United

States. A thunderstorm rumbles on thehorizon. What type is it most likely to be?Why?2. What conditions must be present fora thunderstorm to form?3. Explain why a cold-front thunderstorm isusually more severe than a warm-frontthunderstorm.4. Thinking Critically In the tropics, wherethe tropopause is higher than in otherareas, cumulonimbus clouds commonlyreach towering heights of 15 000 m. Whyis the height of the tropopause a factor inhow tall a cumulonimbus cloud can grow?

earthgeu.com/self_check_quiz

SKILL REVIEW5. Concept Mapping Use the followingphrases to complete an events-chainconcept map about the life cycle ofa thunderstorm. For more help, referto the Skill Handbook.1. waterdropletscoalesce

updraftsstop

3. surfaceair is warmed byconduction

4.precipitationends

5.water vaporcondenses

6. warm airrises, creatingupdrafts

7. surfaceair is cooled bydowndrafts

8. latentheat is released

9. precipitationand downdraftsbegin

2.

13.1 Thunderstorms 333

13.2OBJECTIVES

Severe Weather

Recognize the dangers of

All thunderstorms are not created equal. Some die out within minutes,while others flash and thunder throughout the night. What makesone thunderstorm more severe than another? Occasionally, weatherevents come together in such a way that there is a continuous supplyof surface moisture. This happens along a cold front that moves intowarmer territory and can lift and condense a continuous supply ofwarm air. In this case, a line of thunderstorms can last for hours oreven days as they continually regenerate themselves with the new,warm air that is introduced into the updrafts.

Describe how tornadoes

form.

SEVERE THUNDERSTORMS

Explain why some thunderstorms are more severe

than others.

Other factors also play a role in causing some storms to be more

severe than others. Cold fronts are usually accompanied by upperlevel, low-pressure systems that are marked by pools of cold air. Thiscold, high air increases the temperature difference between the upperand lower parts of the storm, which causes the air to become moreunstable. As the instability of the air increases, the strength of thestorms updrafts and downdrafts intensifies. The storm is then considered to be severe. Severe thunderstorms can produce some of themost violent weather conditions on Earth. They may develop intoself-sustaining, extremely powerful storms called supercells, whichare characterized by intense, rotating updrafts. Figure 13-4B showsan illustration of a supercell. These furious storms can last for severalhours and can have updrafts as strong as 240 km/h.

VOCABULARY

supercelldownbursttornadoFujita tornado intensityscale

Figure 13-4 An anvilshaped cumulonimbus cloud

is characteristic of manysevere thunderstorms (A).The most severe thunderstorms are supercells (B).

Anvil

Rain-free base

334 CHAPTER 13 The Nature of Storms

Precipitation

++

Steppedleader

++ ++ +

++ ++ ++

++

++

Returnstroke

Figure 13-5 When a

stepped leader nears anobject on the ground, apowerful surge of electricityfrom the ground movesupward to the cloud,and lightning is produced.

Channel

Source: NOAA

Of the estimated 100 000 thunderstorms that occur each year in

the United States, only about ten percent are considered to be severe,and fewer still reach classic supercell proportions. But when certainatmospheric conditions come together in the right way, the resultscan be spectacular and sometimes deadly, as youll learn next.

LIGHTNINGHave you ever touched a metal object on a dry winter day and beenzapped by a spark of electricity? If so, you were, in a sense, playingwith lightning. Lightning is electricity caused by the rapid rush of airin a cumulonimbus cloud. A lightning bolt forms when frictionbetween the updrafts and downdrafts within a cumulonimbus cloudseparates electrons from some of their atoms either in the cloud ornear the ground. The atoms that lose electrons become positivelycharged ions. Other atoms receive the extra electrons and becomenegatively charged ions. As Figure 13-5 shows, this creates regions ofair with opposite charges. To relieve the electrical imbalance, an invisible channel of negatively charged air, called a stepped leader, movesfrom the cloud toward the ground. When the stepped leader nears theground, a channel of positively charged ions, called the return stroke,rushes upward to meet it. The return stroke surges from the ground tothe cloud, illuminating the channel with about 100 million V of electricity. That illumination is lightning. Youll learn more about lightning in the Science & Technology feature at the end of this chapter.The Power of Lightning A lightning bolt heats the surroundingair to about 30 000C. Thats about five times hotter than the surfaceof the Sun! The thunder you hear is the sound produced as this superheated air rapidly expands and contracts. Because sound waves travel13.2 Severe Weather 335

Table 13-1 Thunderstorm and Lightning Safety

When Thunderstorms Approach . . . Remember: If you can hear thunder, you are close enough to the storm to bestruck by lightning. Go to a safe shelter immediately. Move to a sturdy building or car. Do not take shelter in small sheds, underisolated trees, or in convertible automobiles. If lightning is occurring and a sturdy shelter is not available, get inside a hardtopped automobile and keep the windows up. Get out of boats and away from water. Telephone lines and metal pipes can conduct electricity. Unplug appliancesnot necessary for obtaining weather information. Avoid using any electricalappliances. Use phones ONLY in an emergency.If You Are Caught Outdoors and No Shelter Is Nearby . . . Find a low spot away from trees, fences, and poles. Make sure the place youchoose is not subject to flooding. If you are in the woods, take shelter under the shorter trees. If you feel your skin tingle or your hair stand on end, squat low to theground on the balls of your feet. Place your hands on your knees with yourhead between them. Make yourself the smallest target possible, andminimize your contact with the ground.Source: NOAA

more slowly than light waves, you may see lightning well before youhear thunder, even though they are generated at the same time.Each year in the United States, lightning accounts for about 7500forest fires, which result in the loss of millions of acres of forest. Inaddition, lightning strikes in the United States cause a yearly averageof 300 injuries and 93 deaths to humans. Table 13-1 lists safetytips to follow to avoid property damage and loss of life from lightning strikes.

THE FURY

OF THE

WIND

Recall that rain-cooled downdrafts descend to Earths surface during

a thunderstorm and spread out as they reach the ground. Sometimes,however, instead of dispersing that downward energy over a largearea underneath the storm, the energy becomes concentrated in alocal area. The resulting winds are exceptionally strong, with speedsof more than 160 km/h. Violent downdrafts that are concentrated ina local area are called downbursts.Based on the size of the area they affect, downbursts are furtherclassified as either macrobursts or microbursts. Macrobursts can causea path of destruction up to 5 km wide. They have wind speeds of morethan 200 km/h and can last up to 30 minutes. Smaller in size, though

336 CHAPTER 13 The Nature of Storms

deadlier in force, microbursts affect areas of less than 3 km but can

have winds exceeding 250 km/h. Despite lasting less than 10 minuteson average, a microburst is especially deadly because its smaller sizemakes it extremely difficult to detect and thus prepare for.

Figure 13-6 This car was

damaged by large hailstones similar to the oneshown here.

HAILEach year in the United States, almost $1 billion in damage is causedby another danger associated with thunderstorms: hail. Hail is precipitation in the form of balls or lumps of ice. It can do tremendousdamage to crops, particularly in the Central United States, where hailoccurs most frequently. Hail is most common during the springgrowing season. Figure 13-6 shows the damage that hail can cause.Hail forms because of two characteristics common to thunderstorms. First, water droplets exist in the liquid state in the parts of acumulonimbus cloud where the temperature is actually below freezing. When these supercooled water droplets encounter ice pellets, thewater droplets freeze on contact and cause the ice pellets to growlarger. The second characteristic that allows hail to form is an abundance of strong updrafts and downdrafts existing side by side withina cloud. The growing ice pellets are caught alternately in the updraftsand downdrafts, so that they are constantly encountering moresupercooled water droplets. The ice pellets keep growing until theyare too heavy for even the strongest updrafts to keep aloft, and theyfinally fall to Earth as hail.

Figure 13-7 This rural community was devastated by

a flood in Arizona.

FLOODSSometimes, the wind currents in the upper atmosphere that causeweather systems to move are weak, and the weather systems andresulting storms move slowly. When this happens, a storm may dumpits rain over a limited location, rather than spreading it over a largearea. Floods such as the one in Figure 13-7 can occur. The situationcan worsen if there is abundant moisture available not just at Earthssurface, but also throughout the atmosphere. This makes the wholeprocess of condensation, coalescence, and precipitation much more13.2 Severe Weather 337

efficient and thus produces more rainfall. If the rain falls faster thanthe ground can absorb it, or faster than streams and rivers can transport it out of the area, flooding can occur. Floods are the main causeof thunderstorm-related deaths in the United States each year.

TORNADOES

Figure 13-8 A change in

wind direction and speedcreates a horizontal rotation in the lower atmosphere (A). Strong updraftstilt the rotating air froma horizontal to a verticalposition (B). A tornadoforms within the rotatingwinds (C).

Of all the dangers associated with thunderstorms, the most impressive by far is a tornado. A tornado is a violent, whirling column ofair in contact with the ground. Before a tornado reaches the ground,it is called a funnel cloud. Tornadoes are often associated with supercells, the most severe thunderstorms. The air in a tornado is madevisible by dust and debris drawn into the swirling column, or by thecondensation of water vapor into a visible cloud. Over the area itcovers, few storms on Earth can match a tornados violence.A tornado forms when wind speed and direction change suddenlywith height, a phenomenon known as wind shear. Under the rightconditions, this can produce a horizontal rotation near Earths surface, as shown in Figure 13-8. If this rotation takes place closeenough to the thunderstorms updrafts, the twisting column of windcan be tilted from a horizontal to a vertical position. As updraftsaccelerate the rotation, air is removed from the center of the column,which in turn lowers air pressure in the center. The extreme pressuregradient between the center and the outer portion of the tornadoproduces the violent winds associated with tornadoes. Although tornadoes rarely exceed 200 m in diameter and usually last only a fewminutes, they can be extremely destructive. In fact, they are classifiedaccording to their destructive force.Tornado Classification Tornadoes can vary greatly in size andintensity. They are classified according to the Fujita tornadointensity scale, which ranks tornadoes according to their path ofdestruction, wind speed, and duration. The Fujita scale was named

338 CHAPTER 13 The Nature of Storms

for Japanese tornado researcher Dr. Theodore

Fujita. The scale ranges from F0, which ischaracterized by winds of up to 118 km/h, tothe incredibly violent F5, which can packwinds of more than 500 km/h. Most tornadoes do not exceed the F1 category. In fact,only about one percent ever reach the violentcategories of F4 and F5. Those that do, however, can lift entire buildings from their foundations and toss automobiles and trucksaround like toys. The Fujita scale is shown inTable 13-2.Tornado Distribution While tornadoescan occur at any time and at any place, thereare some times and locations that are moreconducive to their formation. Most tornadoesespecially the violent onesform inthe spring during the late afternoon andevening, when the temperature contrastsbetween polar air, which still has winter characteristics, and tropical air, which is steadilybecoming warmer, are the greatest. These largetemperature contrasts often spark the development of supercells, which are each capableof producing several strong tornadoes. Largetemperature contrasts occur most frequentlyin the Central United States, where cold continental polar air collides with maritimetropical air moving northward from the Gulfof Mexico. More than 700 tornadoes touchdown each year in the United States. Many ofthese occur in a region called Tornado Alley,which extends from northern Texas throughOklahoma, Kansas, and Missouri.

Table 13-2 Fujita Scale

Weak Tornadoes (F0 and F1)80% of all tornadoesPath: up to 3 milesWind speed: 60 to 115 mphDuration: 110 minutes +

Strong Tornadoes (F2 and F3)

Violent Tornadoes (F4 and F5)

Tornado Safety In the United States, an

average of 80 deaths and 1500 injuries resultfrom tornadoes each year. In an ongoingeffort to reduce tornado-related fatalities,the National Weather Service issues tornadowatches and warnings before a tornado actually strikes. These advisories are broadcaston local radio stations when tornadoes are

13.2 Severe Weather 339

Table 13-3 Tornado Safety

If a Warning Is Issued or If Threatening Weather Approaches . . . If you are in a home or building, move to a predesignated shelter, such as abasement. If an underground shelter is not available, move to an interior room or hallway on the lowest floor and get under a sturdy piece of furniture. Stay away from windows. Get out of automobiles. Do not try to outdistance a tornado in a car; instead, leave the car immediately. If you are caught outside or in a vehicle, lie flat in a nearby ditch or depression. Mobile homes, even when tied down, offer little protection from tornadoesand should be abandoned.Source: NOAA

indicated on weather radar or spotted in the region. During a severe

thunderstorm, the presence of dark, greenish skies, a towering wall ofclouds, large hailstones, and a loud, roaring noise similar to that of afreight train are signs of an approaching or developing tornado. Table13-3 lists safety measures recommended by the National WeatherService in the event of a tornado. The agency stresses that despiteadvanced tracking systems, some tornadoes develop exceedinglyquickly. In these cases, advance warnings may not be possible.However, the threat of tornado-related injury can be substantiallydecreased when people seek shelter at the first sign of threatening skies.In the next section, youll learn about another type of severe weather:tropical storms.

1. Describe two characteristics of thunderstorms that lead to hail formation.

2. Compare and contrast a macroburst anda microburst.3. What type of front would you expect tobe associated with flooding? Why?4. Why are some thunderstorms more severethan others?5. If the time between when you see lightning and hear thunder is increasing, astorm is moving away from you. Why isthis true?

340 CHAPTER 13 The Nature of Storms

6. Thinking Critically Based on what you

know about stepped leaders and returnstrokes, why are tall objects more likely tobe struck by lightning than shorter ones?

SKILL REVIEW7. Recognizing Cause and Effect In theUnited States, most thunderstorms occurin Florida, yet the central states experience the strongest tornadoes. Whydoesnt Florida have more violent tornadoes? For more help, refer to the SkillHandbook.

earthgeu.com/self_check_quiz

13.3

Tropical Storms

If you wanted to search for the origin of the most violent type ofstorm on Earth, the last place youd probably look would be thecalm, sunny tropics. However, during summer and fall, the sunnytropics are the birthing grounds of large, rotating, low-pressurestorms called tropical cyclones. The strongest of these cyclonicstorms are known in the United States and other parts of theAtlantic Ocean as hurricanes. Figure 13-9 illustrates the rotatingnature of a typical hurricane.

TROPICAL CYCLONESUnlike midlatitude storms that derive their energy from the contrastbetween warm and cold air masses, tropical cyclones thrive on thetremendous amount of energy in warm, tropical oceans. As waterevaporates from the ocean surface, latent heat is stored. This latentheat is later released when the air begins to rise and water vapor condenses into clouds and rain. The air usually rises because of somesort of existing weather disturbance moving across the tropics. Manysuch disturbances originate along the Intertropical ConvergenceZone (ITCZ), which you learned about in Chapter 12. As these disturbances produce more precipitation, more energy is released. Inaddition, the rising air creates an area of low pressure at the oceansurface. As more warm air moves toward the low-pressure center toreplace the air that has risen, the Coriolis effect causes the moving airto turn counterclockwise in the northern hemisphere. This producesthe cyclonic rotation of a tropical cyclone.As the moving air approaches the center of the growing storm, itrises, rotates faster and faster, and increases in speed as more energyis released through condensation. In the process, air pressure in thecenter of the system continues to decrease, while surface wind speeds

Figure 13-9 The characteristic rotating nature of

Hurricane Breeding Grounds

North Pacific Ocean

North Atlantic Ocean

South Pacific Ocean

Figure 13-10 Hurricanes

form in all of Earths tropical oceans except in the relatively cool waters of theSouth Pacific and SouthAtlantic Oceans.

South Atlantic Ocean

Indian Ocean

increasesometimes in excess of 240 km/h. This process will continue as long as atmospheric conditions allow warm air to be fed intothe system at the surface and to be removed from the system in theupper atmosphere.Formation of Tropical Cyclones Tropical cyclones requiretwo basic conditions to form: an abundant supply of very warmocean water and some sort of disturbance to lift warm air and keepit rising. These conditions exist in all tropical oceans except theSouth Atlantic Ocean and the Pacific Ocean west of the SouthAmerican Coast. Ocean waters in these areas are somewhat cooler. Inaddition, the ITCZ is positioned farther north. As a consequence,tropical cyclones do not occur in these areas. They do occur in thelarge expanse of warm waters in the western Pacific Ocean, wherethey are known as typhoons. To people living near the Indian Ocean,they are known as cyclones. Near the Atlantic Ocean, the CaribbeanSea, the Gulf of Mexico, and along the western coast of Mexico, theyare called hurricanes. The map in Figure 13-10 shows where hurricanes generally form. They occur most frequently in the late summerand early fall, when Earths oceans contain their greatest amount ofstored heat energy.

342 CHAPTER 13 The Nature of Storms

Movement of Tropical Cyclones Like all large-scale storms,

tropical cyclones move according to the wind currents that steerthem. Recall that many of the worlds oceans are home to subtropical high-pressure systems that are present to some extent throughoutthe year. In the deep tropics, tropical cyclones are often caught up inthe circulation of these high-pressure systems. They move steadilytoward the west, then eventually turn poleward when they reach thefar edges of the high-pressure systems. There, they are guided by prevailing westerlies and begin to interact with midlatitude systems. Atthis point, the interaction of the various wind and weather systemsmakes the movement of the storms unpredictable.Stages of Tropical Cyclones A traveling tropical disturbance,which can cause air in a developing tropical cyclone to rise, is thefirst stage of a tropical cyclone. Disturbances can originate eitherfrom the ITCZ or as weak, low-pressure systems called tropicalwaves. These disturbances are common during the summer andearly fall. Sometimes, midlatitude weather disturbances can moveinto the tropics, become stranded there, and gradually acquire tropical characteristics. Whatever their origin, only a small percentage oftropical disturbances ever develop into full-fledged hurricanes. Thisis because conditions throughout the atmosphere must be such thatrising air can be dispersed into the upper atmosphere. Figure 13-11shows a cross section of a hurricane.

Figure 13-11 In this hurricane cross section, the rising, moist airindicated bysmall red arrowsformsclouds in bands around theeye. The photo shows theeye of a hurricane thatformed over the PacificOcean in 1991.

Descending air

Eyewall

Eye

Warm, moist air

13.3 Tropical Storms 343

Table 13-4 Saffir-Simpson Hurricane Scale

ScaleSustainedNumberWinds(Category)(mph)

Damage

Examples of Hurricanesand the States Affected

7495

Minimal

Florence, 1988 (LA)

Charley, 1988 (NC)

96110

Moderate

Kate, 1985 (FL Panhandle)

Bob, 1991 (RI)

111130

Extensive

Alicia, 1983 (N. TX)

Emily, 1993 (NC Outer Banks)

131155

Extreme

Andrew, 1992 (S. FL)

Hugo, 1989 (SC)

> 155

Catastrophic

Camille, 1969 (LA/MS)

Labor Day Hurricane, 1935 (FL Keys)

Source: National Weather Service

Using Numbers

Suppose that a hurricane has been spotted at 25N, 50W,

which is roughly2900 km from Miami,Florida. The hurricane is moving westat 25 km/h. How longwill it take the hurricane to reach Miami?

When a disturbance over a tropical ocean acquires a cyclonic circulation around a center of low pressure, it has reached the nextdevelopmental stage, which is known as a tropical depression. Whenwind speeds around the low-pressure center of a tropical depressionexceed 65 km/h, the system is called a tropical storm. If air pressurecontinues to fall and winds around the center reach at least 120km/h, the storm is officially classified as a hurricane. Once windsreach these speeds, another phenomenon takes placethe development of a calm center of the storm called an eye. The strongest windsin a hurricane are usually concentrated in a band immediately surrounding the eye called the eyewall.

CLASSIFYING HURRICANESThe Saffir-Simpson hurricane scale classifies hurricanes accordingto wind speed, air pressure in the center, and potential for propertydamage. As shown in Table 13-4, the Saffir-Simpson hurricane scaleranges from Category 1 hurricanes, which have minimum windspeeds of 74 mph (120 km/h), to the monstrous Category 5 storms,which can have winds in excess of 155 mph (250 km/h). Once a hurricane reaches Category 3 status, it is considered to be a majorhurricane, with good reason. Most of the deadliest hurricanes thatstrike the United States were classified as major hurricanes.Running Out of Energy A hurricane will last until it can nolonger produce enough energy to sustain itself. This usually happenswhen the storm moves over land and no longer has access to thewarm ocean surface from which it draws its energy, or when the

344 CHAPTER 13 The Nature of Storms

storm moves over colder water. During its life cycle, a hurricane canundergo several fluctuations in intensity as it interacts with otheratmospheric systems.

HURRICANE HAZARDSHurricanes can cause a lot of damage, particularly along coastal areaswhere human populations have increased. Much of this damage isassociated with violent winds. The strongest winds in a hurricane areusually confined to the eyewall, the band about 40 to 80 km wide thatsurrounds the calm eye. Outside of the eyewall, winds taper off withdistance from the center, although winds of more than 60 km/h canextend as far as 400 km from the center of a hurricane.Storm Surges Strong winds moving onshore in coastal areas arepartly responsible for another major hurricane threat: storm surges.A storm surge occurs when hurricane-force winds drive a mound ofocean water toward coastal areas, where it washes over the land.Storm surges can sometimes reach 6 m above normal sea level, asshown in Figure 13-12. When this occurs during high tide, the surgecan cause enormous damage. In the northern hemisphere, a stormsurge occurs primarily on the right side of a storm relative to its eye,where the strongest onshore winds occur.The heat released through the condensation of vast amounts ofwater vapor fuels hurricanes. This condensation also produces greatamounts of rain. Thus, floods are an additional hurricane hazard,particularly if the storm moves over mountainous areas, where orographic lifting enhances the upward motion of air.

Mean sea level

6.6 m storm tide

0.6 m normal high tide

13.3 Tropical Storms 345

Table 13-5 Hurricane Safety

Turn the refrigerator to the maximum cold settingand open it only when necessary. Turn off utilities if told to do so by authorities. Unplug small appliances.

Turn off propane tanks.

Fill bathtubs and large containers with water forsanitary purposes.

If Winds Become Strong . . .

Stay away from windows and doors even if they are covered. Take refuge in a small interior room, closet, or hallway. Close all interior doors. Secure and brace external doors. If you are in a two-story house, go to an interior first-floor room, such as a bathroom or closet. If you are in a multiple-story building and away from water, go to the first or second floor and take refuge in ahall or other interior room away from windows. Lie on the floor under a table or other sturdy object.Source: NOAA

Hurricane Advisories The National Hurricane Center, which is

responsible for tracking and forecasting the intensity and motion oftropical cyclones in the western hemisphere, issues a hurricane warning at least 24 hours before a hurricane strikes. The center also issuesregular advisories that indicate a storms position, strength, andmovement. Using this information, people can then track a storm ona hurricane-tracking chart, such as the one youll use in the InternetGeoLab at the end of this chapter. This type of awareness, combinedwith proper safety precautions such as those listed in Table 13-5, hasgreatly reduced death tolls associated with hurricanes in recent years.

1. Identify the four main stages of a tropical

cyclone.2. Describe the changing wind systems thatguide a tropical cyclone as it moves fromthe tropics to the midlatitudes.3. Why dont tropical cyclones form in theSouth Atlantic Ocean or off the westerncoast of South America?4. What two conditions must exist for atropical cyclone to form?5. Thinking Critically Suppose that youlive on the eastern coast of the UnitedStates and are advised that the center

346 CHAPTER 13 The Nature of Storms

of a hurricane is moving inland 70 km

north of your location. Would you predictthat a storm surge will be a major problem in your area? Why or why not?

SKILL REVIEW6. Making and Using Tables Research atleast ten hurricanes that have occurredthroughout the world since 1980. Basedon the Saffir-Simpson scale, make a datatable showing wind speed, air pressure inthe center, and property damage associated with each hurricane. For more help,refer to the Skill Handbook.

earthgeu.com/self_check_quiz

Environmental Connection

13.4

Recurring Weather

On a hot, summer day, a sudden thunderstorm is a welcome

event. Such rains are not so welcome, however, when they continuefor hours or even days. Persistent or repetitive weather can negativelyaffect agriculture, transportation, and recreation.

FLOODS

AND

DROUGHTS

An individual thunderstorm can unleash enough rain to produce

floods, and hurricanes are notorious for their torrential downpours.Floods can also occur, however, when weather patterns cause evenmild storms to persist over the same area. For example, a storm witha rainfall rate of 1.5 cm/h is not much of a problemproviding thatit lasts only an hour or two. If this same storm were to remain overone spot for 18 hours, however, total rainfall would be 27 cm, whichis more than enough to create flooding in most areas. You will learnmore about floods in the MiniLab on the following page.On the other hand, too much dry weather can create nearly asmuch havoc as too much rainfall. Droughts are extended periods ofwell-below-normal rainfall. One of the most extreme droughtsoccurred during the 1930s in the Central United States. Figure 13-13shows a dust storm that occurred in the Dust Bowl, which was thename given to the affected states. This extended drought put countless farmers out of business, as rainfall for several seasons was inadequate to grow crops.

Figure 13-13 The dust

blowing over this highwaywas caused by a severedrought in the CentralUnited States during the1930s.

13.4 Recurring Weather 347

How can mild

rains cause floods?Model the effects of repeated, slow-movingstorms that drop rain over the same area fora long period of time.

Procedure1. Place an ice-cube tray on the bottom ofa large sink or tub.2. Pour water into a clean, plastic dishwashingdetergent bottle until it is two-thirds full.Replace the cap on the bottle.3. Hold the bottle upside down with the capopen about 8 cm above one end of theice-cube tray. Gently squeeze the bottle tomaintain a constant flow of water intothe tray. Slowly move the bottle from oneend of the tray to the other over thecourse of 30 seconds. Try to put approximately equal amounts of water in eachice-cube compartment.4. Measure the depth of water in each compartment. Calculate the average depth.5. Repeat steps 14, but move the bottleacross the ice-cube tray in 15 seconds.

Analyze and Conclude

1. How did the average depth of the waterdiffer in steps 4 and 5? How might youaccount for the difference?2. Based on these results, infer how thespeed of a moving storm affects theamount of rain received in any one area.3. How could you alter the experiment tosimulate different rates of rainfall?

348 CHAPTER 13 The Nature of Storms

Droughts are usually the result of shifts

in global wind patterns that allow largehigh-pressure systems to persist for weeksor months over continental areas. Under adome of high pressure, air sinks on a largescale. Because the sinking air will resist anyattempt to lift moisture through it, condensation cannot occur, and drought will set inuntil global patterns shift enough to movethe high-pressure system out of the way.Heat Waves An unpleasant side effect ofdroughts often comes in the form of heatwaves, which are extended periods ofabove-normal temperatures. Heat wavescan be formed by the same high-pressuresystems that cause droughts. As the airunder a large high-pressure system sinks, itwarms by compression and causes abovenormal temperatures. The high-pressuresystem also blocks cooler air masses frommoving into the area, so there is little relieffrom the heat. Because it is difficult for condensation to occur under the sinking air ofthe high-pressure system, there are few, ifany, clouds to block the blazing sunshine. Tomake matters worse, the jet stream, oratmospheric railway, that weather systemsfollow is farther north and weaker duringthe summer. Thus, the upper-air currentsthat might guide the high-pressure systemare so weak that the system scarcely moves.Even increasing humidity does not easethe discomfort of a heat wave. Human bodies cool by evaporating moisture from thesurface of the skin. In the process, heat isremoved from the body. If air is humid, therate of evaporation is reduced, whichdiminishes the bodys ability to regulateinternal temperature. In heat waves, this canlead to serious health problems such asheatstroke, sunstroke, and even death.

Table 13-6 The Heat Index

Air Temperature (F)Relative70Humidity(%)

75

80

85

90

95

100

105

110

115

120

Apparent Temperature (F)

64

69

73

78

83

87

91

95

99

103

107

10

65

70

75

80

85

90

95

100

105

111

116

20

66

72

77

82

87

93

99

105

112

120

130

30

67

73

78

84

90

96

104

113

123

135

148

40

68

74

79

86

93

101

110

123

137

151

50

69

75

81

88

96

107

120

135

150

60

70

76

82

90

100

114

132

149

70

70

77

85

93

106

124

144

80

71

78

86

97

113

136

90

71

79

88

102

122

100

72

80

91

108

Source: National Weather Service, NOAA

Because of the extreme dangers posed by the lethal combination of

heat and humidity, the National Weather Service routinely reports theheat index, shown in Table 13-6. Note that the National WeatherService uses the Fahrenheit scale in the heat index because most U.S.citizens are most familiar with this scale. The heat index assesses theeffect of the bodys increasing difficulty in regulating its internaltemperature as relative humidity rises. For example, an air temperature of 85F (29C) combined with relative humidity of 80 percentwould require the body to cool itself at the same rate as if the air temperature were 97F (36C). Do the Problem-Solving Lab on the following page to learn more about heat waves.

COLD WAVESThe flip side of a heat wave is a cold wave, which is an extendedperiod of below-normal temperatures. Interestingly, cold waves arealso brought on by large, high-pressure systems. However, cold wavesare caused by systems of continental polar or arctic origin. Duringthe arctic winter, little sunlight is available to provide warmth. At thesame time, the snow-covered surface is constantly radiating itslimited heat back to space. The combined effect of these two factorsis the development of large pools of extremely cold air over polarcontinental areas. Because cold air sinks, the pressure near the surface increases, creating a strong high-pressure system.13.4 Recurring Weather 349

Because of the location and the time of year in which they occur,winter high-pressure systems are much more influenced by the jetstream than are summer high-pressure systems. Moved along by thejet stream, these high-pressure systems rarely linger in any area.However, the winter location of the jet stream may remain essentiallyunchanged for days or even weeks. This means that several polarhigh-pressure systems can follow the same path and subject the sameareas to bout after bout of numbing cold. Figure 13-14 shows someeffects of prolonged periods of cold weather.Because wind transports heat away from the body, the effects ofcold air are worsened by wind. This phenomenon is known as thewind-chill factor. The wind-chill factor is measured by the windchill index, which estimates the heat loss from human skin caused bythe combination of cold air and wind. This index estimates how coldthe air actually feels to the human body. As with the heat index, theNational Weather Service records the wind-chill index in U.S. units

Making and Using Graphs

Charting a heat wave The following

2. Plot the daily maximum temperatures

data represent the daily maximum and

minimum temperatures for ten consecutive summer days in a major city.

on a graph with the days on the x-axis

and the maximum temperatures onthe y-axis. Connect the data points toshow how the maximum temperaturechanged over the ten-day period.3. Repeat step 2 for the minimum andaverage temperatures.

Daily TemperaturesDay

Maximum

Minimum

Average

12345678910

92919495939694969294

76757875777680727468

84838685858687848381

Analysis1. Copy the data table in your sciencejournal. Calculate the average temperature for each day, then includethose temperatures in your data table.

350 CHAPTER 13 The Nature of Storms

Thinking Critically4. A heat wave is defined as two or moreconsecutive days with an average temperature of 85F or higher. On whatday did the city begin its heat wave?How long did the heat wave last?5. Calculate the average temperature forthe days of the heat wave only. Compare this to the average temperatureof the remaining days.6. What safety measures could residentsof the city take to minimize the effectsof a heat wave?

BAfor the sake of convenience. While the wind-chill index is helpful, itdoes not account for individual variations in sensitivity to cold, theeffects of physical activity, or humidity. Some scientists, noting thatthis system has been in place since the 1940s, are calling for thedevelopment of new methods that more accurately estimate theeffects of cold weather on the human body.

1. Why are droughts usually associated with

high-pressure systems?2. Describe a situation wherein a relativelylight rain could cause flooding.3. Compare and contrast a cold wave and aheat wave.4. What is the wind-chill factor? What doesthe wind-chill index measure?5. Using Table 13-6, estimate the heat indexfor air with a temperature of 80F andrelative humidity of 90 percent.6. Extreme floods occur more often in summer than any other time of the year. Useyour knowledge of the jet stream toexplain why this is true.

Figure 13-14 An ice storm

7. Thinking Critically Air in a summer highpressure system warms by compression.

Based on what you know about molecularmotion, explain why air in a winter highpressure system doesnt warm bycompression, too.

SKILL REVIEW8. Forming a Hypothesis A key requirementfor the formation of snow is cold air. Yetsome parts of the United States havemore annual snowfall than Canada, whichis farther north and should therefore becolder. Form a hypothesis to explain theapparent discrepancy. For more help,refer to the Skill Handbook.

13.4 Recurring Weather 351

Tracking a Hurricane

urricanes are violent storms. Thats why its important

to have plenty of advance warning before they hit land.By tracking the changing position of a storm on a chart andconnecting these positions with a line, you can determine ahurricanes path.

PreparationProblemWhat information can you obtain bystudying the path of a hurricane?

Plot data on a hurricane-tracking

chart. Predict where storm-inflicted damage

might occur.

HypothesisGather information about the path of ahurricane. Form a hypothesis abouthow the hurricanes path can be used topredict the strength of the storm andwhere most damage might be inflicted.Objectives Gather and communicate data abouthurricanes.

Data SourcesGo to the Earth Science Web siteat earthgeu.com to find links tohurricane data, or use informationprovided by your teacher. Make copiesof the hurricane-tracking chart in thislab or download a chart from theWeb site.

Plan the Experiment

1. Find a resource that lists major hurri-

canes that have occurred within the

past five years. The Earth ScienceWeb site provides a list of sites thathave information about hurricanes.2. Choose a hurricane to research. Some

recent major hurricanes include

Hurricane Claudette, HurricaneIsabel, and Hurricane Floyd.3. Gather data about the hurricane fromthe links on the Earth Science Website or the library.

Procedure1. Incorporate your research into a

data table. Add any additional information that you think is important.2. Go to the Earth Science Web site atearthgeu.com to post your data.352 CHAPTER 13 The Nature of Storms

Conclude & Apply

Sharing Your Data Find this Internet

4. Multiply the value from question 3

GeoLab on the Earth Science Web site at

earthgeu.com. Post your data in thetable provided for this activity. Use theadditional data from other students tocomplete your chart and answer theConclude & Apply questions.1. Plot the position, air pressure, windspeed, and stage of the hurricane atsix-hour intervals throughout itsexistence.2. Plot the changing position of thehurricane on your hurricanetracking chart.3. What was the maximum wind speedin knots that the hurricane reached?

by 1.15 to find the wind speed in

miles per hour. Based on this value,how would the hurricane be classified on the Saffir-Simpson scale?5. Using your completed hurricanetracking chart, list the landmassesover which the hurricane passed.6. Where would you expect the stormsurge to have been greatest? Explain.Compare your answer to the information you gathered on the damageinflicted by the storm. Was youranswer correct?7. How was the hurricanes strengthaffected when its center passed overland?Internet GeoLab 353

Taming LightningIn a high-voltage laboratory in Canada, scientists experiment withlightning on a regular basis. Their goal is to one day work outside, triggering lightning bolts and directing them safely away from people andproperty. They are among a group of scientists worldwide who aredeveloping new and better ways to tame lightning before it strikes.Each year in the United States, lightningaccounts for 7500 forest fires, roughly 93deaths, several hundred injuries, and millions ofdollars in damage to communications equipment,buildings, electrical systems, and power lines.To guard against loss of life and property, mosthomes and commercial structures are equippedwith lightning-protection systems. These systemsuse lightning rodsslender metal rods placedupon rooftopsto gather positive charges fromthe ground. The positive charges attract the negative charges in the base of a thundercloud andneutralize them split seconds before they coalesce into a lightning strike. Aluminum or coppercables act as conducters, connecting the lightning rods to ground terminators, which are metalrods buried beneath the soil. The function ofthese rods is to guide the electrical currentharmlessly into the ground.Lightning-protection systems such as thesehave been around since Benjamin Franklin flewhis kite in a storm some 200 years ago, demonstrating conclusively that lightning is indeed electricity. Today, one of the most promising areas oflightning-protection research involves somethingthat Benjamin Franklin never had at his disposallaser beams.

Home-Grown LightningLets return to the high-voltage lab in Canada.There, scientists use two huge, circular elec-

354 CHAPTER 13 The Nature of Storms

trodes to re-create the natural conditions that

result in a shocking bolt of lightning. One electrode is suspended about 6 m above the other.The top electrode represents the base of a thundercloud. The bottom electrode represents theground. Just as in nature, negative charges flowdown from the thundercloud and positivecharges flow up from the ground. The wave ofcurrent that surges upward to meet the downward discharge results in the bright, jagged flashknown as lightning.The goal of these scientists, however, is notsimply to model lightning. They want to learn howto harness lightningto trigger controlled strikesand guide them to safe locations. To do this, thescientists aim a laser beam through a hole in thecenter of the bottom electrode. The beam displaces electrons from charged particles in theair, simultaneously provoking a lightning strikeand providing a guided path for the discharge tofollow. Instead of a jagged bolt of lightning, thecontrolled strike is as straight as a laser beam.

ActivityWheres the safest place to be during athunderstorm? If youre caught outside inthe open, what should you do? Researchlightning safety tips, then develop a safetybrochure for distribution at your school.

SummarySECTION 13.1Thunderstorms

SECTION 13.2Severe Weather

SECTION 13.3Tropical Storms

SECTION 13.4RecurringWeather

Main Ideas For a thunderstorm to occur, there must be abundant moisturein the lower levels of the atmosphere and a mechanism to liftthe moisture so it can condense. In addition, the air must beunstable so that the growing cloud will continue to rise. Thunderstorms are classified according to the mechanism thatcaused the air to rise. In an air-mass thunderstorm, the cloudrises because of unequal heating of Earths surface within oneair mass. In a frontal thunderstorm, the air rises because it ispushed up by an advancing air mass.

Main Ideas Lightning is produced when an advancing stepped leader uniteswith an upward-moving return stroke. Thunder is the soundmade by the rapid expansion of air around the lightning bolt asa result of extreme heating of the lightning channel. Thunderstorms can damage property and cause loss of life. Thehazards of thunderstorms include lightning, violent winds, hail,floods, and tornadoes. The Fujita tornado intensity scale classifies tornadoes accordingto wind speed, path of destruction, and duration.

Main Ideas Tropical cyclones derive their energy from the evaporation ofwarm ocean water and the release of heat. The Saffir-Simpson hurricane scale classifies hurricanes accordingto intensity. Hurricane hazards include violent winds, floods, and stormsurges. The National Hurricane Center tracks hurricanes andissues advance warnings to help reduce loss of life.

Main Ideas Examples of persistent weather events include floods, droughts,cold waves, and heat waves. The heat index assesses the impact of humidity combined withexcessive heat on the human body. The wind-chill index estimates the heat loss from human skin caused by a combinationof cold air and wind.

Study Guide 355

Understanding Main Ideas

1. Which of the following would work against thedevelopment of a thunderstorm?a. rising airc. moistureb. stable aird. unstable air2. Which of the following does NOT describe a typeof damaging thunderstorm wind?a. downburstc. land breezeb. microburstd. macroburst3. Flooding is most likely to take place because ofrains associated with what type of front?a. stationary frontc. cold frontb. occluded frontd. warm front4. During what stage of a tropical cyclone does aneyewall develop?a. tropical depressionc. hurricaneb. tropical stormd. tropical wave5. What is the first stage of a lightning bolt?a. return strokec. positive chargeb. stepped leaderd. downdraft6. Which of the following does NOT play a key rolein the development of hail?a. supercooled waterc. warm ocean waterb. strong downdraftsd. strong updrafts7. Heat waves involve high-pressure systems thatcause air to sink and warm by which of the following processes?a. compressionc. evaporationb. conductiond. condensation8. Which of the following weather hazards involveslack of moisture?a. hailc. storm surgeb. droughtd. flood

356 CHAPTER 13 The Nature of Storms

9. What percentage of tornadoes are classified as F4

or F5 on the Fujita tornado intensity scale?a. one percentc. 50 percentb. ten percentd. 75 percent10. Which of the following factors, if increased,would make a thunderstorm severe?a. temperaturec. durationb. surface moistured. conduction11. Which way do hurricanes rotate in the southernhemisphere?a. southc. counterclockwiseb. clockwised. north12. In which ocean would you NOT expect to experience a tropical cyclone?a. West Pacificc. North Atlanticb. Indiand. South Atlantic13. What weather events are cold waves most oftenassociated with?a. floodsb. polar high-pressure systemsc. tropical high-pressure systemsd. droughts14. Compare and contrast tornadoes and hurricanes.15. Why are cold fronts more likely to produce severethunderstorms than warm fronts?

Test-Taking TipCAREFULLY OBSERVE SCIENTIFICILLUSTRATIONS If a test question requiresyou to interpret a scientific illustration, look veryclosely at the details of the illustration. Youranswer may depend on a small detail.

earthgeu.com/chapter_test

Standardized Test Practice

Applying Main Ideas

16. Using Table 13-6, determine the heat index ifthe temperature is 90F and relative humidity is60 percent.17. Using Table 13-4, classify a hurricane with amaximum wind speed of 120 mph.Use the illustration of a hurricane in the northernhemisphere to answer question 18.

INTERPRETING SCIENTIFIC ILLUSTRATIONS

Use the illustration below to answer questions1 and 2.+

++ ++ +

++

Point A

++ ++ ++

++

++

Direction ofmotion

1. Which type of cloud is lightning associated

with?a. altocumulusc. cirrusb. stratocumulusd. cumulonimbus

Point B

18. Would a storm surge be more likely to occur at

point A or point B? Why?19. How might you prepare for a tornado? Whatsafety measures would you recommend?20. In which oceans would you NOT expect to experience a tropical cyclone? Why?

Thinking Critically21. Extreme cold waves are more common in thenorthern hemisphere than in the southern hemisphere. Why?22. Tropical cyclones are never observed within about5 north and south latitudes. What do you thinkmight account for this?23. Supercells that produce tornadoes often producelarge hailstones as well. Explain.24. Why are boats on lakes or on the ocean especially vulnerable to lightning strikes?

earthgeu.com/standardized_test

2. Lightning is the illumination that occurs

when an invisible channel of negativelycharged air descends to the ground and achannel of positively charged ions rushesupward to meet it. What is the channel ofpositively charged ions called?a. return strokec. ground strokeb. stepped leader d. electronic leader3. What occurs when winds of at least120 km/h drive a mound of ocean watertoward coastal areas?a. downburstc. storm surgeb. cold waved. tornado4. Which factor is NOT associated with a heatwave?a. a high-pressure systemb. a weakened jet streamc. above-normal temperaturesd. increased cloud cover